Readme: add usage information

README.md

 # Nexenta Exporter for Prometheus
 This Prometheus exporter uses the Nexenta API to provide metrics for JBODs, volumes and license informations.
 
-#### Currently implemented metrics:
+### Usage
+The exporter accepts arguments and/or environment variables to set the credentials and connection details.
+
+Flag | Env Var | Default
+--- | --- | ---
+`-api-host` | `API_HOST` | `nexenta`
+`-api-port` | `API_PORT` | `8457`
+`-api-user` | `API_USER` | `admin`
+`-api-password` | `API_PASSWORD` | `password`
+`-api-ssl` | `API_SSL` | `false`
+`-insecure` | `INSECURE` | `false`
+`-listen-address` | `LISTEN_ADDRESS` | `:9457`
+`-metrics-endpoint` | `METRICS_ENDPOINT` | `/metrics`
+
+### Currently implemented metrics:
 Metric | Type | Description
 --- | --- | ---
 `nexenta_up` | gauge | Is the Nexenta API reachable (0/1)